It is known in the art to perform control to stepwisely increase on-duty ratio of a PWM signal and thereby slowly raise transfer voltage at a time of starting up transfer bias voltage.
However, in the art, when leak current flows from a photoreceptor to an applying circuit that generates transfer bias voltage, it is concerned that the applying circuit can be disabled to start up because of influence of the leak current. Then, with the control as is in the art, the PWM value gradually becomes larger and, therefore, the applying circuit is started up in due course. However, with such control, it is concerned that the duty ratio may be increased too much at the time of start-up thus causing over-current.
Thus, there is a need in the art for an image forming apparatus that can suitably perform start-up of the transfer bias voltage without generating over-current.